1. Field of the Invention
The present invention relates to a multilayer ceramic substrate and a method for producing the same, and more particularly relates to a multilayer ceramic substrate which includes a resistor located on at least one principal surface, and further an overcoat layer located thereon, and a method for producing the same.
2. Description of the Related Art
When a resistor composed of a thick film is formed on a principal surface of a multilayer ceramic substrate, the resistor is overcoated with a glass-based material for the purposes of protecting this resistor and improving environmental resistance. In addition, the resistor is typically subjected to trimming after firing to adjust the resistance value.
For example, Japanese Patent Application Laid-Open No. 5-234726 discloses a method for forming a thick film resistor circuit, which includes the steps of: printing and burning a resistor onto a substrate; printing and burning a conductor in a predetermined position on the resistor; forming an overcoat layer on the resistor; and performing laser trimming to the resistor from the top of the overcoat layer to adjust a resistance value.
However, according to the technique described in Japanese Patent Application Laid-Open No. 5-234726, the substrate, the resistor, the conductor, and the overcoat layer are formed separately by firing or burning, there is thus a problem that the frequent firing or burning promotes sintering of the substrate, resistor, and conductor, thereby resulting in an excessively sintered state, and thus in characteristic degradation.
Therefore, techniques have been known for obtaining a substrate, a resistor, a conductor, and an overcoat layer by co-firing. For example, Japanese Patent Application Laid-Open No. 2005-39164 discloses a method for producing a glass ceramic wiring substrate, which includes the steps of: preparing an unfired ceramic green laminated body of a plurality of glass ceramic green sheets stacked on each other; forming a resistor, a conductor, and an overcoat layer on one principal surface of the ceramic green laminated body; then preparing an unfired composite laminated body by stacking a constraining green sheet along both principal surfaces of the ceramic green laminated body; firing the unfired composite laminated body; and then removing a constraining layer derived from the constraining green sheet.
However, it has been discovered that the following problem is caused in the case of forming the resistor and the overcoat layer by co-firing with the ceramic green laminated body, and then carrying out trimming for the resistor as in the technique described in Japanese Patent Application Laid-Open No. 2005-39164.
FIG. 6 is a cross-sectional view illustrating a portion of a multilayer ceramic substrate 51. FIG. 6 illustrates a ceramic laminated body 52 which is provided for the multilayer ceramic substrate 51, and a resistor 54, a resistor connecting conductor 55 with a portion overlapping with the resistor 54, and an overcoat layer 56 covering the resistor 54, which are each formed on a principal surface 53 of the ceramic laminated body 52.
To explain with reference to FIG. 6, there is a demand for the overcoat layer 56 to be formed as thinly as possible in order to shorten the trimming time after firing. However, when the overcoat layer 56 is formed thinly, the film thickness is likely to vary, and thus, it is difficult to form the overcoat layer 56 so as to have a uniform thickness.
In addition, when the overcoat layer 56 is formed thinly, cracks are likely to be caused in the overcoat layer 56 due to stress generated by factors such as differences in sintering behavior and coefficient of thermal expansion during firing between each of the ceramic laminated body 52, the resistor connecting conductor 55, and the resistor 54, and the overcoat layer 55. In particular, as in the case of portions 57 and 58 surrounded by dashed lines in FIG. 6, cracks are likely to be caused in portions in which the interface between dissimilar materials is present. The above-described cracks cause the problem of decreased reliability, such as variations in the resistance value of the resistor 54 through the ingress of a plating solution below the overcoat layer 56 in a plating step carried out subsequently.